1. Field of the Invention
The present invention concerns a method and apparatus for determining the torque acting at the surface in a drillstring while drilling a borehole and its application to minimize the fluctuations of the torque in the drillstring so as to produce a more uniform motion of the drillstring.
The drillstring is composed of a drillbit and hollow drillpipes screwed end to end to each other to form a long pipe which can be several thousand meters long. The drillstring is rotated from the surface by an electrical motor, the shaft of which is coupled to the drillstring either, for a rotary table drilling system, by gears, a rotary table and a driving pipe of square section, called "kelly", or by gears only in the case of a top drive drilling system. The driller tries to keep the drillstring rotational speed relatively constant, at an optimal speed so as to maximize the rate of penetration of the drillbit. However, due to the relative elasticity of the drillstring and to its friction along the wall of the borehole, the drillbit rotational speed varies and is not the same as the drillstring rotational speed at the surface. This induces torsional vibration in the drillstring.
Minimizing drillstring torsional vibration is highly desirable since it improves the life expectancy of the drillstring due to a reduction in the cyclic loading of the drillpipes, it reduces the probability of having a stuck pipe or a break of a drillpipe ("twist-off"), and it allows a better control of the direction of the well due to a more regular speed of rotation of the drill bit.
2. Prior Art
The usual method which is used to reduce drillstring torsional vibration is to vary the weight on bit and/or the rotational speed of the drillstring on a trial-and-error basis. The disadvantages of the method are that it does not always work, it relies on the driller's experience and the trial-and-error technique can worsen the torsional vibration. In addition, the torsional vibration may be reduced at the expense of the drilling rate of penetration (e.g. in order to reduce the vibration, the required weight on bit and the drillstring rotational speed to give an optimum rate of penetration may not be achievable).
Another method of reducing drillstring torsional vibration is proposed in SPE (Society of Petroleum Engineers) article number 18049, entitled Torque Feedback Used to Cure Slip-Stick Motion, 1988. This method, called "Torque Feedback", proposes to measure the drillstring torque at the surface and to use the measurement as a feedback signal to manipulate the control system of the electrical motor which rotates the drillstring.
This technique is, however, difficult to apply in practice because of the difficulties involved in measuring the drillstring torque. The known method is based on the measurement of the mechanical strain induced in the drillstring by the torque. A torquemeter, comprising strain gauges, is positioned between the rotary table and the kelly. Such a torquemeter is for example described in U.S. Pat. No. 4,471,663. This method of measuring drillstring torque based on mechanical strain measurement is difficult to implement on a drilling rig. Because the drillstring torque has to be measured while the drillstring is rotating, the measurement of the mechanical strain must be transmitted via either a telemetry system or sliprings and brushes. A telemetry system, such as a radio link for example, requires a battery which rotates with the drillstring. The drilling operation must therefore be interrupted periodically for the battery replacement. On the other hand, the reliability and the maintainability of a sliprings/brushes system are questionable in such a severe environment as the top of the drillstring. In addition, the environment around the drillstring, where the torque measurement is required, is subjected to severe mechanical vibration and shock as well as contaminants and debris. Also, due to risk of explosion in that particularly sensitive area of the drilling rig, the electronic equipment must be specially built to comply with compulsory requirements and be certified. It is not easy to find certified electronic sensors and components which satisfy the requirements. Furthermore, in the majority of cases there is insufficient space for mounting a torque sensor within the rotary table.